The Effect of Microstructure on the Toughness of the 0.2C-3Cr-W-Mo-V-Co Precipitation Hardening Hot Work Alloy Tool Steel

Abstract

The effect of bainite structure on the toughness of the 0.2C-3Cr-W-Mo-V-Co steel was investigated.
The results obtained are as follows:
(1) The granular upper bainite structure, which was obtained by quenching at a rate of the half temperature time of 60 min provided the following superior as well as inferior aspects compared to the martensite structure tempered at 400°C. (i) Inferior aspects are lower plane strain fracture toughness and higher transition temperature in charpy impact testing. (ii) Superior aspects are lower fatigue clack propagation rate, higher upper shelf energy in charpy impact testing, and higher uniform elongation in tensile testing.
(2) Microstructural factors dominant to the above behavior on the toughness of bainite could be summarized as follows. (i) Larger width of granular bainite grain and larger size of microstructural unit relevant to the effective grain size increase the cleavage fracture sensitivity. (ii) Excessively stable retained austenite located both along grain boundaries and in grains improves the toughness. (iii) Suppressed precipitation of M₃C carbide improves the toughness.
(3) When this bainite structure was tempered at higher temperatures of 550 to 650°C, this structure showed the lower toughness than the martensite structure because of reasons of higher distribution density of fine MC and M₂C carbide precipitates, etc.